1. Field of the Invention
The present invention relates to a substrate having inside an electrically conductive layer (internal electrically conductive layer), having an electrically conductive via hole formed between the internal electrically conductive layer and the surface of the substrate, which is formed from an aluminum nitride sintering product, and relates to a process for producing the substrate. More particularly, the invention relates to the above-mentioned substrate which has high thermal conductivity, good adhesion properties between the aluminum nitride sintering product and the internal electrically conductive layer or the electrically conductive via hole and high denseness of the internal electrically conductive layer and the electrically conductive via hole and hardly suffers warpage, and relates to a process for producing the substrate.
2 . Description of the Prior Art
Since aluminum nitride sintering products have excellent properties such as high thermal conductivity, good electrical insulation properties and a coefficient of thermal expansion almost equal to that of Silicon (Si) for forming integrated circuits, they are used as substrates (sometimes referred to as xe2x80x9caluminum nitride substratesxe2x80x9d hereinafter) on which semiconductor circuit parts are mounted. Particularly, aluminum nitride substrates having a so-called electrically conductive via hole (sometimes referred to as a xe2x80x9cvia holexe2x80x9d simply hereinafter) that is a through hole filled with an electrically conductive material enable electrical connection between external circuits of a semiconductor through the via hole, and hence they are extremely useful.
In recent years, miniaturization of semiconductor circuit manufactured articles and improvement of performance thereof have been promoted, and with such promotion, the number of the via holes formed in the aluminum nitride substrates has been increased and the arrangement of the via holes has been complicated. To meet such market requirements, there has been utilized such an aluminum nitride substrate (1) as shown in the sectional view of FIG. 1, which has inside an internal electrically conductive layer (2) and plural electrically conductive via holes (3) which are electrically connected to one another through the internal electrically conductive layer. The aluminum nitride substrate having an electrically conductive layer inside can be produced by laminating plural aluminum nitride molded products having via holes through electrically conductive paste layers and then dewaxing and sintering the resulting aluminum nitride molded product laminate.
The aluminum nitride sintering product produced by the above process, however, has a thermal conductivity of at most about 170 W/mK at 25xc2x0 C. because of restriction imposed by that sintering of the electrically conductive layer and sintering of the substrate are simultaneously carried out.
On the other hand, as one method to calcine aluminum nitride (simple substrate) having no via hole and no conductive layer, a two-step firing method wherein the sintering temperature is changed in the specific range is carried out (Japanese Patent Laid-Open Publication No. 105525/1993). In this method, a high thermal-conductive aluminum nitride sintering product having a thermal conductivity of about 200 W/mK at 25xc2x0 C. can be obtained.
When the sintered aluminum nitride substrate having the via hole and the internal electrically conductive layer is prepared according to the above-mentioned two-step firing method, however, it was difficult to obtain sufficiently high adhesion strength between the aluminum nitride sintering product and the internal electrically conductive layer. Further, there was a problem that the electrically conductive layer did not have satisfactory denseness and as a result cracks occurred inside the aluminum nitride substrate or the value of resistance of the via hole was increased. Moreover, there was a problem of large warpage of the substrate.
Accordingly, it has been desired to develop an aluminum nitride substrate with a via hole and an internal electrically conductive layer, which has a high thermal conductivity of the aluminum nitride sintering product, high adhesion strength of the internal electrically conductive layer or the via hole to the aluminum nitride sintering product and other excellent properties.
The present inventors have found that the defects of the substrate are caused by the action of carbon after dewaxing, and further found the following: cracks occurring inside the aluminum nitride substrate and increase of warpage of the substrate can be inhibited by controlling the carbon residue of the dewaxed aluminum nitride molded product laminate to the specific range; the adhesion strength between the aluminum nitride sintering product and the internal electrically conductive layer or the via hole can be sufficiently enhanced and stabilized by controlling the carbon residue of the dewaxed laminate, the composition of the electrically conductive paste used and the temperature range of the two-step firing method to the specific ranges; and the thermal conductivity of the aluminum nitride sintering product can also be sufficiently enhanced by these operations.
The substrate according to the invention is a substrate having an internal electrically conductive layer, at least one electrically conductive via hole formed between the internal electrically conductive layer and at least one surface of the substrate, which is formed from an aluminum nitride sintering product, wherein:
the thermal conductivity of the aluminum nitride sintering product at 25xc2x0 C. is 190 W/mK or more, and the adhesion strength between the aluminum nitride sintering product and the internal electrically conductive layer is 5.0 kg/mm2 or more.
In the substrate of the invention, the adhesion strength between the aluminum nitride sintering product and the electrically conductive via hole is preferably 5.0 kg/mm2 or more.
In the substrate of the invention, it is preferable that the electrically conductive via hole comprises a sintering product of an electrically conductive paste having a refractory metal concentration of 85 to 95% by weight and the internal electrically conductive layer comprises a sintering product of an electrically conductive paste having a refractory metal concentration of 65 to 83% by weight.
A metallized substrate according to the invention has an electrically conductive pattern formed on at least one surface of both surfaces of the above-mentioned substrate, wherein at least a part of the electrically conductive pattern is electrically connected to the electrically conductive via hole.
The process for producing a substrate according to the invention comprises:
forming at least one via hole-forming through hole in a first aluminum nitride molded product comprising an aluminum nitride powder, a sintering aid and an organic binder,
filling the through hole with an electrically conductive paste (A) comprising 100 parts by weight of a refractory metal powder and 2 to 10 parts by weight of an aluminum nitride powder,
coating the surface of the first aluminum nitride molded product with an electrically conductive paste (B) comprising 100 parts by weight of a refractory metal powder and 2 to 20 parts by weight of an aluminum nitride powder to form an electrically conductive paste layer,
laminating a second aluminum nitride molded product comprising an aluminum nitride powder, a sintering aid and an organic binder on the first aluminum nitride molded product through the layer of the electrically conductive paste (B), and
dewaxing the resulting aluminum nitride molded product laminate so that the carbon residue becomes 800 to 3000 ppm, then sintering the laminate at a temperature of 1200 to 1700xc2x0 C. and further sintering the laminate at a temperature of 1800 to 1950xc2x0 C.
In the above process, it is preferable that the concentration of the refractory metal in the electrically conductive paste (A) with which the via hole-forming through hole of the first aluminum nitride molded product is to be filled is in the range of 85 to 95% by weight and the concentration of the refractory metal in the electrically conductive paste (B) with which the surface of the first aluminum nitride molded product is to be coated is in the range of 65 to 83% by weight.
It is particularly preferable that the viscosity of the electrically conductive paste (A) with which the via hole-forming through hole of the first aluminum nitride molded product is to be filled is in the range of 100 to 30000 poise at 25xc2x0 C./5 rpm and the viscosity of the electrically conductive paste (B) with which the surface of the first aluminum nitride molded product is to be coated is in the range of 800 to 1200 poise at 25xc2x0 C./5 rpm.
According to the process of the invention, the aforesaid substrate of the invention can be produced.